Question

In which of the following pairs of compounds is the first molecule more reactive than the second molecule towards both $S_{N}1$ and $S_{N}2$?

• A. II and IV
• B. I, II and III
• C. I, II, III and IV
• D. III only

Answer 1

The correct option is B I, II and III


I) Rate of $S_{N}1$ and $S_{N}2$ for first molecule will be greater than the second. In $S_{N}1$ reaction, the carbocation formed can be stabilized by the lone pair of oxygen by resonance. In $S_{N}2$ reaction, oxygen increases the electrophilicity of the reactive site by inductive effect.


II) Rate of $S_{N}1$ and $S_{N}2$ for first molecule will be greater than the second. In $S_{N}1$ reaction, the carbocation formed can be stabilized by the double bond by resonance. In $S_{N}2$ reaction, $s{p}^2$ carbon increases the electrophilicity of the reactive site by inductive effect.


III) Rate of $S_{N}1$ and $S_{N}2$ for the first molecule will be greater than the second. In $S_{N}1$ reaction, the carbocation formed can be stabilized by resonace of benzene ring. In $S_{N}2$ reaction, -R effect of benzene increases the electrophilicity of the reactive site. Also haloarenes are not reactive towards nucleophilic substitution.


IV) First molecule is more reactive towards $S_{N}1$, since it is a tertiary substrate. The second molecule will be more reactive than the first one towards $S_{N}2$ reaction.